Opera. Closed Source and relys on small dev team that work on the Mobile client. Updates, Vunerabilities, Bugs, etc. all must come from this team at their own time frame.

Firefox. Open source. Infinite number of people can be working on optimizing and adding new features to this browser for the best experience possible because of it's open source nature.

When the new firefox browser comes out on the new N series I don't think you'll be missing your opera browser much and even if you do you'll most likely still be able to keep it.